Frequency modulation circuit, transmission circuit, and communication apparatus

ABSTRACT

A bandpass type delta sigma modulation section performs delta sigma modulation on an inputted modulation signal such that quantization noise is reduced in a frequency band which requires low noise. A low pass filter removes a noise component in a high frequency region from the signal on which the delta sigma modulation has been performed. A frequency modulation circuit reduces noise in the frequency band which requires low noise with the bandpass type delta sigma modulation section and the low pass filter, and reduces noise in the vicinity of a direct current component DC with a feedback comparison section and a loop filter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a frequency modulation circuit which isused in communication apparatuses such as mobile phones, wireless LANdevices, or the like, a transmission circuit, and a communicationapparatus. More particularly, the present invention relates to afrequency modulation circuit which reduces noise present in a desiredfrequency band and operates with low distortion and high efficiency, atransmission circuit using the frequency modulation circuit, and acommunication apparatus.

2. Description of the Background Art

Communication apparatuses such as mobile phones, wireless LAN devices,or the like need to operate with low power consumption while maintainingthe accuracy of an output signal. Such communication apparatuses requirea frequency modulation circuit which outputs a frequency modulationsignal having low distortion and operates with high efficiency and atransmission circuit using the frequency modulation circuit. Thefollowing will describe a conventional frequency modulation circuit.

As an example of a conventional frequency modulation circuit, thereexists a frequency modulation circuit which is disclosed in Wendell B.Sander, et al, “Polar Modulator for Multi-mode Cell Phones”, US,Tropian, Inc. FIG. 13 is a block diagram showing an exemplarilyconfiguration of a conventional frequency modulation circuit 500. Asshown in FIG. 13, the conventional frequency modulation circuit 500includes first and second arithmetic units 507 and 508, a loop filter502, a voltage controlled oscillator (VCO) 503, a frequency digitalconverter (FDC) 504, a DA converter (DAC) 505, and a low pass filter(LPF) 506.

A modulation signal is inputted to the conventional frequency modulationcircuit 500 through an input terminal. The modulation signal is inputtedto the VCO 503 through the first arithmetic unit 507, the secondarithmetic unit 508 and the loop filter 502. The modulation signal isalso inputted to the VCO 503 through the DAC 505 and the LPF 506. TheVCO 503 frequency-modulates the modulation signal by controlling anoscillatory frequency according to the inputted modulation signal, andoutputs a resultant signal as a frequency modulation signal.

The second arithmetic unit 508 adds or subtracts a constant to or fromthe inputted modulation signal to control the center frequency of themodulation signal. FDC 504 converts the frequency of the frequencymodulation signal outputted by the VCO 503 into a digital value, andoutputs the converted digital value to the first arithmetic unit 507.The first arithmetic unit 507 adds the digital value outputted by theFDC 504 to the modulation signal outputted by the second arithmetic unit508, and outputs a resultant signal to the loop filter 502. The loopfilter 502 suppresses the high-frequency component of the modulationsignal outputted by the first arithmetic unit 507. In other words, theFDC 504, the first arithmetic unit 507, and the loop filter 502constitute a feedback loop which stabilizes the frequency of thefrequency modulation signal outputted by the VCO 503.

Meanwhile, the DAC 505 converts the inputted modulation signal into ananalog signal. The LPF 506 serves to suppress noise such as quantizationnoise which is generated with the processing by the DAC 505.

FIG. 14 illustrates a problem of the conventional frequency modulationcircuit 500. FIG. 14 shows a relationship between noise and a frequencywhich are included in each of an output signal Ax of the DAC 505, anoutput signal Bx of the LPF 506 when the LPF 506 is not connected to aninput of the VCO 503, and an input signal Cx to VCO 503 when the LPF 506is connected to the input of the VCO 503. It is noted that the symbolsAx to Cx in FIG. 14 correspond to the points Ax to Cx in FIG. 13,respectively.

As shown in FIG. 14, the output signal Ax of the DAC 505 includessubstantially uniform quantization noise and the like. The LPF 506removes noise of a high frequency region from the output signal Ax ofthe DAC 505. Since the LPF 506 is connected to the input of the VCO 503,the output signal Bx of the LPF 506 is inputted as the input signal Cxto the VCO 503 after noise in the vicinity of a direct current componentDC is removed by the operation of the feedback loop of the conventionalfrequency modulation circuit 500.

As shown in FIG. 14, however, the noise of the input signal Cx to theVCO 503 is not always sufficiently reduced in a frequency band whichrequires low noise (the shaded area in FIG. 14). Therefore, theconventional frequency modulation circuit 500 has a problem that itcannot output a frequency modulation signal the noise of which issufficiently reduced in the frequency band which requires low noise.

There is a possibility that in the case where the conventional frequencymodulation circuit 500 is installed in a communication apparatus whichperforms transmission and reception concurrently, the noise generated atthe conventional frequency modulation circuit 500 overlaps with areceiving band of the communication apparatus, and this adverselyaffects the receiving quality of the communication apparatus.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a frequencymodulation circuit which reduces the noise present in the frequency bandrequiring low noise and operates with low distortion and highefficiency, a transmission circuit using the frequency modulationcircuit, and a communication apparatus.

The object of the present invention is directed to a frequencymodulation circuit which frequency-modulates and outputs an inputsignal. In order to achieve the above-mentioned object, the frequencymodulation circuit of the present invention comprises a bandpass typedelta sigma modulation section operable to perform delta sigmamodulation on the input signal such that quantization noise issuppressed in a frequency band which requires low noise; a loop filteroperable to output a signal which is the input signal a high-frequencycomponent of which is suppressed; a voltage controlled oscillatoroperable to frequency-modulate the input signal by controlling aoscillatory frequency according to the signal outputted from the loopfilter and the bandpass type delta sigma modulation, and output aresultant signal as a frequency modulation signal; and a feedbackcomparison section operable to compare the frequency modulation signalwhich is fed back from the voltage controlled oscillator with the inputsignal, control a frequency of the input signal according to the resultof the comparison, and output the frequency-controlled input signal tothe loop filter.

It is noted that the frequency modulation circuit may comprise acompensating filter operable to filter and output the input signal tothe bandpass type delta sigma modulation section to compensate outputcharacteristics of the bandpass type delta sigma modulation section.

Preferably, the feedback comparison section includes a frequency digitalconverter operable to convert a frequency of the frequency modulationsignal outputted by the voltage controlled oscillator into a digitalvalue based on a predetermined manner; and an arithmetic sectionoperable to add or subtract the digital value converted by the frequencydigital converter to or from the input signal.

Preferably, the frequency modulation circuit may comprise a lowpass typedelta sigma modulation section operable to perform delta sigmamodulation with signal transfer characteristics of lowpass type on thesignal outputted from the loop filter, and output to the voltagecontrolled oscillator the signal on which the delta sigma modulation hasbeen performed. In this case, the frequency modulation circuit mayfurther comprise a second arithmetic section operable to input to thearithmetic section a signal a center frequency of which is controlled byadding or subtracting a constant to or from the input signal.

The frequency modulation circuit may further comprise a lowpass typedelta sigma modulation section operable to perform delta sigmamodulation with signal transfer characteristics of lowpass type on theinput signal, and output to the feedback comparison section the signalon which the delta sigma modulation has been performed, and the feedbackcomparison section may include a frequency divider operable tofrequency-divide the frequency modulation signal outputted by thevoltage controlled oscillator with the signal on which the delta sigmamodulation has been performed by the lowpass type delta sigma modulationsection; and a comparison section operable to compare a predeterminedreference signal with the signal which has been frequency-divided by thefrequency divider, and control the oscillatory frequency of the voltagecontrolled oscillator such that both of the signals are synchronized.

The present invention is directed to a transmission circuit includingthe above-mentioned frequency modulation circuit. The transmissioncircuit comprises a signal generation section operable to generate anamplitude signal and a phase signal based on an amplitude component anda phase component which are obtained by performing signal processing oninput data; an amplitude amplifying section operable to output a signalwhich is controlled according to the amplitude signal; the frequencymodulation circuit operable to frequency-modulate and output the phasesignal as a frequency modulation signal, and an amplitude modulationsection operable to amplitude-modulate the frequency modulation signalby using the signal outputted from the amplitude amplifying section, andoutput the frequency-modulated and amplitude-modulated signal as atransmission signal.

Further, the present invention is directed to a communication apparatusincluding the above-mentioned transmission circuit. The communicationapparatus comprises the transmission circuit operable to generate atransmission signal; and an antenna operable to output the transmissionsignal generated by the transmission circuit. The communicationapparatus may comprise a reception circuit operable to process areception signal received from the antenna, and an antenna duplexeroperable to output the transmission signal generated by the transmissioncircuit to the antenna, and output the reception signal received fromthe antenna to the reception circuit.

According to the above-mentioned present invention, noise is reduced inthe frequency band which requires low noise by the bandpass type deltasigma modulation section, and noise in the vicinity of a direct currentcomponent DC are further reduced by the feedback loop. Thus, a frequencymodulation signal the noise of which is reduced can be outputted in thefrequency band which requires low noise.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an exemplary configuration of a frequencymodulation circuit 1 according to a first embodiment of the presentinvention;

FIG. 2 illustrates characteristics of a bandpass type delta sigmamodulation section 15;

FIG. 3 illustrates the operation of the frequency modulation circuit 1according to the first embodiment of the present invention;

FIG. 4 is a diagram showing a detailed configuration of the bandpasstype delta sigma modulation section 15;

FIG. 5 is a diagram showing an exemplary configuration of a frequencymodulation circuit 2 according to a second embodiment of the presentinvention;

FIG. 6A illustrates an example of output characteristics of each sectionof the frequency modulation circuit 2 when the function of acompensating filter 21 is OFF;

FIG. 6B illustrates an example of output characteristics of each sectionof the frequency modulation circuit 2 when the function of thecompensating filter 21 is ON;

FIG. 7 is a diagram showing an exemplary configuration of a frequencymodulation circuit 3 a according to a third embodiment of the presentinvention;

FIG. 8 illustrates characteristics of a lowpass type delta sigmamodulation section 35;

FIG. 9 is a diagram showing another exemplary configuration of afrequency modulation circuit 3 b according to the third embodiment ofthe present invention;

FIG. 10 is a diagram showing an exemplary configuration of a frequencymodulation circuit 4 according to a fourth embodiment of the presentinvention;

FIG. 11 is a diagram showing an exemplary configuration of atransmission circuit 5 according to a fifth embodiment of the presentinvention;

FIG. 12 is a diagram showing an exemplary configuration of acommunication apparatus 200 according to a sixth embodiment of thepresent invention;

FIG. 13 is a diagram showing an exemplary configuration of aconventional frequency modulation circuit 500; and

FIG. 14 illustrates a problem of the frequency modulation circuit 500.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 1 is a block diagram showing an exemplary configuration of afrequency modulation circuit 1 according to a first embodiment of thepresent invention. As shown in FIG. 1, the frequency modulation circuit1 according to the first embodiment comprises a feedback comparisonsection 11, a loop filter 12, a voltage controlled oscillator (VCO) 13,a bandpass type delta sigma modulation section 15, and a low pass filter(LPF) 16.

A modulation signal is inputted to the frequency modulation circuit 1through an input terminal. The modulation signal is inputted to the VCO13 through the feedback comparison section 11 and the loop filter 12.The modulation signal is also inputted to the VCO 13 through thebandpass type delta sigma modulation section 15 and LPF 16. The VCO 13frequency-modulates the modulation signal by controlling an oscillatoryfrequency based on the inputted modulation signal, and outputs aresultant signal as a frequency modulation signal.

The feedback comparison section 11 returns and inputs the frequencymodulation signal outputted by the VCO 13. The feedback comparisonsection 11 compares the frequency of the inputted modulation signal withthe frequency of the returned frequency modulation signal, and outputsto the loop filter 12 the modulation signal the frequency of which iscontrolled according to the result of the comparison. The loop filter 12suppresses the high-frequency component of the controlled modulationsignal outputted from the feedback comparison section 11. In otherwords, the feedback comparison section 11, the loop filter 12, and theVCO 13 constitute a feedback loop which stabilizes the frequency of thefrequency modulation signal outputted by the VCO 13.

The bandpass type delta sigma modulation section 15 performs delta sigmamodulation on the inputted modulation signal, and outputs a resultantsignal as a delta sigma modulation signal. A detailed configuration ofthe bandpass type delta sigma modulation section 15 is shown in FIG. 4.In the case of the configuration in FIG. 4, noise transfer function is“1+2Z⁻²+Z⁻⁴” having a zero point at π/2 and low noise at a frequency offs/4. The LPF 16 suppresses noise such as quantization noise generatedwith the processing by the bandpass type delta sigma modulation section15.

FIG. 2 shows characteristics of the bandpass type delta sigma modulationsection 15 in FIG. 1. As shown in FIG. 2, the bandpass type delta sigmamodulation section 15 has signal transfer characteristics of bandpasstype, and has noise characteristics reverse to the signal transfercharacteristics due to the effect of noise shaping. Thus, thecharacteristics of the bandpass type delta sigma modulation section 15is designed such that quantization noise is minimized in a frequencyband (the shaded area in FIG. 2) which requires low noise by the deltasigma modulation.

FIG. 3 illustrates the operation of the frequency modulation circuit 1according to the first embodiment of the present invention. FIG. 3 showsa relationship between noise and a frequency which are included in eachof an output signal A of the bandpass type delta sigma modulationsection 15, an imaginary output signal B of the LPF 16 when the outputof the LPF 16 is not connected to the input of the VCO 13 (hereinafterreferred to merely as the output signal B of the LPF 16), an inputsignal C to the VCO 13 when the output of the LPF 16 is connected to theinput of the VCO 13 (hereinafter referred to merely as the output signalC to the VCO 13). It is noted that the symbols A to C in FIG. 3correspond to the points A to C in FIG. 1, respectively.

The modulation signal inputted through the input terminal becomes theoutput signal A the quantization noise of which is minimized in afrequency band (the shaded area in FIG. 3) which requires low noise bythe optimally-designed bandpass type delta sigma modulation section 15.The output signal A becomes the output signal B such that the noise ofthe output signal A in a high frequency region including at least thefrequency band which requires low noise is removed by the LPF 16. Theoutput signal B becomes the input signal C the noise of which in thevicinity of a direct current component DC is reduced by the operation ofthe feedback loop of the frequency modulation circuit 1, and isoutputted to the VCO 13.

Therefore, the VCO 13 changes the oscillatory frequency according to theinput signal C and outputs a frequency modulation signal noise of whichis reduced in the frequency band which requires low noise. As seen fromFIG. 3, since the conventional frequency modulation circuit 500 withoutthe bandpass type delta sigma modulation section 15 cannot sufficientlyreduce quantization noise in the frequency band which requires lownoise, noise characteristics (the dotted line in FIG. 3) of the inputsignal Cx inputted to the VCO 13 is extremely inadequate in comparisonto the present invention.

As mentioned above, according to the frequency modulation circuit 1 ofthe first embodiment of the present invention, the noise is reduced inthe frequency band which requires low noise by the bandpass type deltasigma modulation section 15, and the noise in the vicinity of the directcurrent component DC is reduced by the feedback loop. Thus, thefrequency modulation circuit 1 can output the frequency modulationsignal the noise of which is reduced in the frequency band whichrequires low noise. When installed in a communication apparatus whichperforms transmission and reception concurrently, the frequencymodulation circuit 1 reduces noise present in a band which overlaps witha receiving band of the communication apparatus, thereby preventingnoise generated at the frequency modulation circuit 1 from adverselyaffecting the receiving quality of the communication apparatus.

Second Embodiment

Although it is ideal to design the bandpass type delta sigma modulationsection 15 so as to obtain desired output characteristics as describedin the above first embodiment, this design is practically difficult.

A second embodiment will describe a frequency modulation circuit whicheven though the bandpass type delta sigma modulation section 15 is notdesigned so as to obtain a desired output characteristics, makes totalcharacteristics close to a desired value by compensating the outputcharacteristics.

FIG. 5 is a block diagram showing an exemplary configuration of afrequency modulation circuit 2 according to the second embodiment of thepresent invention. As shown in FIG. 5, the frequency modulation circuit2 according to the second embodiment comprises a feedback comparisonsection 11, a loop filter 12, a VCO 13, a compensating filter 21, abandpass type delta sigma modulation section 15, and a LPF 16. Thefrequency modulation circuit 2 differs from the frequency modulationcircuit 1 according to the above-mentioned first embodiment in that thecompensating filter 21 is provided in the previous stage of the bandpasstype delta sigma modulation section 15. The compensating filter 21serves to compensate the output characteristics of the bandpass typedelta sigma modulation section 15. It is noted that the same componentsas those in the above-mentioned first embodiment are designated by thesame reference numerals, and the description thereof will be omitted.

The operation of the compensating filter 21 will be described usingFIGS. 6A and 6B. FIG. 6A illustrates an example of outputcharacteristics of each section of the frequency modulation circuit 2when the function of the compensating filter 21 is OFF. FIG. 6Billustrates an example of output characteristics of each section of thefrequency modulation circuit 2 when the function of the compensatingfilter 21 is ON. It is noted that FIGS. 6A and 6B show a relationshipbetween power and a frequency which are included in each of an outputsignal E of the compensating filter 21, an output signal A of thebandpass type delta sigma modulation section 15, an imaginary outputsignal B of the LPF 16 when the output of the LPF 16 is not connected tothe input of the VCO 13, an input signal C to VCO 13 when the output ofthe LPF 16 is connected to the input of the VCO 13.

Since the bandpass type delta sigma modulation section 15 is notdesigned so as to obtain desired output characteristics, the bandpasstype delta sigma modulation section 15 has characteristics which cannotcover a low frequency side like the output signals A indicated by thelong dashed double-short dashed lines in FIGS. 6A and 6B. Meanwhile, thecharacteristics caused by the feedback loop cannot cover a highfrequency side like output signals D indicated by the long dashed shortdashed lines in FIGS. 6A and 6B. There is a frequency region where thecharacteristics of the output signals A and D do not overlap with eachother and which cannot be covered by the output signals A and D.Therefore, the signal power of the input signal C inputted to the VCO 13has characteristics in which a low frequency side is attenuated asindicated by the heavy line in FIG. 6A.

The compensating filter 21 filters the inputted modulation signal suchthat the characteristics on the low frequency band side, which cannot becovered by the bandpass type delta sigma modulation section 15, areraised in advance in order to compensate the output characteristics ofthe bandpass type delta sigma modulation section 15 (the output signal Ein FIG. 6B). The signal power of the input signal C inputted to the VCO13 can obtain characteristics in which the low frequency band side iseven as shown in FIG. 6B by the operation of the compensating filter 21even when the bandpass type delta sigma modulation section 15 is notdesigned so as to obtain desired output characteristics.

As mentioned above, according to the frequency modulation circuit 2 ofthe second embodiment of the present invention, even when the bandpasstype delta sigma modulation section 15 is not designed so as to obtaindesired output characteristics, the compensating filter 21 compensatesthe output characteristics of the bandpass type delta sigma modulationsection 15. Thus, the frequency modulation circuit 2 can output afrequency modulation signal the noise of which is reduced in thefrequency band which requires low noise.

Third Embodiment

FIG. 7 is a block diagram showing an exemplary configuration of afrequency modulation circuit 3 a according to a third embodiment of thepresent invention. As shown in FIG. 7, the frequency modulation circuit3 a according to the third embodiment comprises a second arithmetic unit32, a feedback comparison section 11, a loop filter 12, a lowpass typedelta sigma modulation section 35, a VCO 13, a bandpass type delta sigmamodulation section 15, an LPF 16 and an LPF 36. The feedback comparisonsection 11 includes a first arithmetic unit 31 and a frequency digitalconverter (FDC) 34. It is noted that the same components as those in theabove-mentioned first embodiment are designated by the same referencenumerals, and the description thereof will be omitted.

A modulation signal (Δθ/Δt, θ is a phase modulation signal) is inputtedto the second arithmetic unit 32 through an input terminal. The secondarithmetic unit 32 adds or subtracts a constant to or from the inputtedmodulation signal to control the center frequency of the modulationsignal. The lowpass type delta sigma modulation section 35 performsdelta sigma modulation on the modulation signal outputted by the loopfilter 12, and outputs to the VCO 13 the modulation signal on which thedelta sigma modulation has been performed. The lowpass type delta sigmamodulation section 35 has signal transfer characteristics of lowpasstype, and reduces noise in the vicinity of a direct current component DCof the modulation signal by performing the delta sigma modulation on themodulation signal. FIG. 8 illustrates characteristics of the lowpasstype delta sigma modulation section 35. The LPF 36 suppresses noise suchas quantization noise generated with the processing by the lowpass typedelta sigma modulation section 35.

The FDC 34 converts the frequency of the frequency modulation signaloutputted by the VCO 13 into a digital value according to apredetermined manner, and outputs the converted digital value to thefirst arithmetic unit 31. The first arithmetic unit 31 adds or subtractsthe digital value outputted by the FDC 34 to or from the modulationsignal inputted through the second arithmetic unit 32, and outputs aresultant signal to the loop filter 12. In other words, the FDC 34, thefirst arithmetic unit 31, the loop filter 12, and the VCO 13 constitutea feedback loop which stabilizes the frequency of the frequencymodulation signal outputted by the VCO 13.

As mentioned above, according to the frequency modulation circuit 3 a ofthe third embodiment of the present invention, the center frequency ofthe frequency modulation signal outputted by the VCO 13 is changed bycontrolling the center frequency of the modulation signal by the secondarithmetic unit 32. The provision of the lowpass type delta sigmamodulation section 35 in the previous stage of the VCO 13 can moreeffectively reduce the noise in the vicinity of the direct currentcomponent DC of the modulation signal. Thus, the frequency modulationcircuit 3 a can output a frequency modulation signal the noise of whichis reduced in the frequency band which requires low noise.

It is noted that in the case where frequency characteristics requiredfor the LPF 16 corresponds to or approximates that for the LPF 36, theLPF 16 and the LPF 36 may be integrated as an LPF 37 like theconfiguration of a frequency modulation circuit 3 b as shown in FIG. 9.Sharing the LPF can reduce a circuit size.

Fourth Embodiment

FIG. 10 is a block diagram showing an exemplary configuration of afrequency modulation circuit 4 according to a fourth embodiment of thepresent invention. As shown in FIG. 10, the frequency modulation circuit4 according to the fourth embodiment comprises a lowpass type deltasigma modulation section 45, a feedback comparison section 11, a loopfilter 12, a VCO 13, a bandpass type delta sigma modulation section 15,and an LPF 16. The feedback comparison section 11 includes a frequencydivider 41 and a comparison section 42. It is noted that the samecomponents as those in the above-mentioned first embodiment aredesignated by the same reference numerals, and the description thereofwill be omitted.

A modulation signal (Δθ/Δt, θ is a phase modulation signal) and areference signal are inputted to the frequency modulation circuit 4through two input terminals, respectively. The modulation signal isinputted to the lowpass type delta sigma modulation section 45 and thebandpass type delta sigma modulation section 15. The lowpass type deltasigma modulation section 45 performs delta sigma modulation with signaltransfer characteristics of lowpass type (cf. FIG. 8) on the inputtedmodulation signal, and outputs to the frequency divider 41 themodulation signal on which the delta sigma modulation has beenperformed. The frequency divider 41 frequency-divides the frequencymodulation signal outputted by the VCO 13 with the modulation signaloutputted from the lowpass type delta sigma modulation section 45. Thecomparison section 42 compares the frequency of the inputted referencesignal with the frequency of the frequency modulation signal inputtedthrough the frequency divider 41, and controls the oscillatory frequencyof the VCO 13 such that both of the signals are synchronized. In otherwords, the frequency divider 41, the comparison section 42, and the loopfilter 12 constitute a feedback loop.

As mentioned above, according to the frequency modulation circuit 4 ofthe fourth embodiment of the present invention, the center frequency ofthe frequency modulation signal outputted by the VCO 13 can be changedby controlling the oscillatory frequency of the VCO 13 using thefeedback loop. Thus, the frequency modulation circuit 4 can output afrequency modulation signal the noise of which is reduced in thefrequency band which requires low noise.

The first to fourth embodiments have described the frequency modulationcircuits 1 to 4 including the LPF 16 and/or the LPF 36. However, whenquantization noise generated at the bandpass type delta sigma modulationsection 15 and/or the lowpass type delta sigma modulation sections 35and 45 are sufficiently reduced, or in the case of a system which has noproblem even when noise is large in a frequency bandwhich is distantfrom the frequency band which requires low noise, the LPF 16 and/or theLPF 36 can be omitted.

Naturally, the compensating filter 21 mentioned in the above secondembodiment can be added in the previous stage of the bandpass type deltasigma modulation section 15 in the frequency modulation circuits 3 a, 3b, and 4 of the third and fourth embodiments.

Fifth Embodiment

FIG. 11 is a block diagram showing an exemplary configuration of atransmission circuit 5 according to a fifth embodiment of the presentinvention. As shown in FIG. 11, the transmission circuit 5 according tothe fifth embodiment comprises a signal generation section 51, afrequency modulation circuit 52, a regulator 54, and an amplitudemodulation section 55. Any one of the frequency modulation circuits 1 to4 mentioned in the first to fourth embodiments is used as the frequencymodulation circuit 52.

The signal generation section 51 generates an amplitude signal and aphase signal from an input signal. The amplitude signal is inputted tothe regulator 54. Direct-current voltage is supplied from a power supplyterminal 53 to the regulator 54. The regulator 54 supplies to theamplitude modulation section 55 a signal which is controlled accordingto the inputted amplitude signal. The phase signal is inputted to thefrequency modulation circuit 52. The frequency modulation circuit 52frequency-modulates the inputted phase signal, and outputs a frequencymodulation signal. The frequency modulation signal is inputted to theamplitude modulation section 55. The amplitude modulation section 55amplitude-modulates the frequency modulation signal with the signalsupplied from the regulator 54, and outputs a resultant signal as afrequency-modulated and amplitude-modulated modulation signal. Themodulation signal is outputted as a transmission signal through anoutput terminal 56. Such transmission circuit 5 is referred to as apolar modulation circuit.

Such transmission circuit 5 operates with low distortion and highefficiency and can operate with low noise since the frequency modulationcircuit 52 outputs a frequency modulation signal the noise of which isreduced in the frequency band which requires low noise. Thus, wheninstalled in a communication apparatus which performs transmission andreception concurrently, the transmission circuit 5 reduces noise presentin a band which overlaps with a receiving band of the communicationapparatus, thereby preventing noise generated at the frequencymodulation circuit 52 from adversely affecting the receiving quality ofthe communication apparatus.

Sixth Embodiment

FIG. 12 is a block diagram showing an exemplary configuration of acommunication apparatus 200 according to a sixth embodiment of thepresent invention. Referring to FIG. 12, the communication apparatus 200according to the sixth comprises a transmission circuit 210, a receptioncircuit 220, an antenna duplexer 230, and an antenna 240. Thetransmission circuit 5 mentioned in the above fifth embodiment is usedas the transmission circuit 210.

The antenna duplexer 230 transmits to the antenna 240 a transmissionsignal outputted from the transmission circuit 210, and prevents thetransmission signal from leaking to the reception circuit 220. Theantenna duplexer 230 transmits to the reception circuit 220 a receptionsignal inputted from the antenna 240, and prevents the reception signalfrom leaking to the transmission circuit 210. Accordingly, thetransmission signal is outputted from the transmission circuit 210, andreleased from the antenna 240 to the exterior space via the antennaduplexer 230. The reception signal is received by the antenna 240 andthen received by the reception circuit 220 via the antenna duplexer 230.

The communication apparatus 200 according to the sixth embodiment usesthe transmission circuit 5 according to the fifth embodiment, therebyensuring the linearity of the transmission signal and achieving lowdistortion of a radio device. Since there is no branch, such as adirectional coupler, on the output of the transmission circuit 210, lossfrom the transmission circuit 210 to antenna 240 can be reduced. Thus,power consumption is reduced at the time of transmission, with theresult that the communication apparatus 200 is capable of operating as aradio communication device for a long period of time. Further, thetransmission circuit 210 of the communication apparatus 200 reducesnoise present in a band which overlaps with a receiving band of thereception circuit 220, thereby preventing noise generated at thetransmission circuit 210 from adversely affecting the receiving qualityof the reception circuit 220. It is noted that the communicationapparatus 200 may comprise only the transmission circuit 210 and theantenna 240.

While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It isunderstood that numerous other modifications and variations can bedevised without departing from the scope of the invention.

1. A frequency modulation circuit for frequency modulating andoutputting an output frequency modulation signal, the frequencymodulation circuit comprising: a bandpass type delta sigma modulationsection operable to receive an input modulation signal and perform deltasigma modulation on the received input modulation signal, such thatquantization noise is suppressed in a frequency band which requires lownoise, and output a first resultant signal; a feedback comparisonsection operable to receive the same input modulation signal, comparethe output frequency modulation signal with the received inputmodulation signal, control a frequency of the received input modulationsignal according to a result of the comparison, and output afrequency-controlled modulation signal; a loop filter operable tosuppress a high-frequency component of the frequency-controlledmodulation signal and output a second resultant signal; and a voltagecontrolled oscillator operable to perform frequency-modulation bycontrolling an oscillatory frequency according to a combination of thesecond resultant signal outputted from the loop filter and the firstresultant signal outputted from the bandpass type delta sigma modulationsection, and output the output frequency modulation signal.
 2. Thefrequency modulation circuit according to claim 1, further comprising acompensating filter operable to filter the input modulation signal andoutput the input modulation signal on which filtering by thecompensating filter has been performed to the bandpass type delta sigmamodulation section to compensate for output characteristics of thebandpass type delta sigma modulation section.
 3. The frequencymodulation circuit according to claim 1, wherein the feedback comparisonsection includes: a frequency digital converter operable to convert afrequency of the output frequency modulation signal outputted by thevoltage controlled oscillator into a digital value based on apredetermined manner; and an arithmetic section operable to add orsubtract the digital value converted by the frequency digital converterto or from the input modulation signal.
 4. The frequency modulationcircuit according to claim 2, wherein the feedback comparison sectionincludes: a frequency digital converter operable to convert a frequencyof the output frequency modulation signal outputted by the voltagecontrolled oscillator into a digital value based on a predeterminedmanner; and an arithmetic section operable to add or subtract thedigital value converted by the frequency digital converter to or fromthe input modulation signal.
 5. The frequency modulation circuitaccording to claim 3, further comprising a lowpass type delta sigmamodulation section operable to perform delta sigma modulation withsignal transfer characteristics of lowpass type on the second resultantsignal outputted from the loop filter, and output to the voltagecontrolled oscillator the second resultant signal on which the deltasigma modulation has been performed.
 6. The frequency modulation circuitaccording to claim 4, further comprising a lowpass type delta sigmamodulation section operable to perform delta sigma modulation withsignal transfer characteristics of lowpass type on the second resultantsignal outputted from the loop filter, and output to the voltagecontrolled oscillator the second resultant signal on which the deltasigma modulation has been performed.
 7. The frequency modulation circuitaccording to claim 5, further comprising a second arithmetic sectionoperable to control a center frequency of the input modulation signal byadding or subtracting a constant to or from the input modulation signal,and output to the arithmetic section the controlled input modulationsignal.
 8. The frequency modulation circuit according to claim 6,further comprising a second arithmetic section operable to control asignal a center frequency of the input modulation signal by adding orsubtracting a constant to or from the input modulation signal, andoutput to the arithmetic section the controlled input modulation signal.9. The frequency modulation circuit according to claim 1, furthercomprising a lowpass type delta sigma modulation section operable toperform delta sigma modulation with signal transfer characteristics oflowpass type on the input modulation signal, and output to the feedbackcomparison section the input modulation signal on which the delta sigmamodulation has been performed, wherein the feedback comparison sectionincludes: a frequency divider operable to frequency-divide the outputfrequency modulation signal outputted by the voltage controlledoscillator with the input modulation signal on which the delta sigmamodulation has been performed by the lowpass type delta sigma modulationsection; and a comparison section operable to compare a predeterminedreference signal with the frequency-divided output frequency modulationsignal from the frequency divider, and control the oscillatory frequencyof the voltage controlled oscillator such that the frequency-dividedoutput frequency modulation signal and the predetermined referencesignal are synchronized.
 10. The frequency modulation circuit accordingto claim 2, further comprising a lowpass type delta sigma modulationsection operable to perform delta sigma modulation with signal transfercharacteristics of lowpass type on the input modulation signal, andoutput to the feedback comparison section the input modulation signal onwhich the delta sigma modulation has been performed, wherein thefeedback comparison section includes: a frequency divider operable tofrequency-divide the output frequency modulation signal outputted by thevoltage controlled oscillator with the input modulation signal on whichthe delta sigma modulation has been performed by the lowpass type deltasigma modulation section; and a comparison section operable to compare apredetermined reference signal with the frequency-divided outputfrequency modulation signal from the frequency divider, and control theoscillatory frequency of the voltage controlled oscillator such that thefrequency-divided output frequency modulation signal and thepredetermined reference signal are synchronized.